The invention relates to marine seismic prospecting and, more particularly, to a device attached to an underwater cable by a sacrificial latch mechanism.
In marine seismic exploration, a streamer cable is towed underwater behind a survey vessel. An array of hydrophones is mounted in the cable, and a seismic source, or gun, is fired to generate seismic energy that radiates through the water and into the earth. Reflections of the seismic energy off geologic strictures are sensed by the hydrophones. Data representing the detected energy are transmitted to the vessel for later processing to produce maps showing subsea formations likely to contain oil and gas.
To create accurate maps, it is necessary to know the shape of the streamer cable during the seismic data-gathering process. For this purpose, depth control devices, commonly referred to as cable-leveling "birds," are attached to the cable at intervals along its length. The depth control devices are equipped with adjustable diving planes, or wings, whose angles of attack can be varied by motor to maintain a preselected depth. The depth control devices frequently include highly accurate heading sensors, or digital compasses, whose heading outputs are transmitted to the vessel. From the compass heading data, the shape of the cable can be accurately determined.
Typically, each depth control device is detachably connected to the cable by one or more collars, which are attached to the cable. Each collar comprises cylindrical inner and outer races surrounding the streamer. The inner race is affixed to the streamer. The outer race, to which the depth control device is semi-pennanently attached, can freely rotate about the inner race and, hence, the streamer. The outer race, which may be hinged for easy removal from and attachment to the inner race, has a slot for engagement by a locking bolt extending out of the depth control device to retain the device to the outer race. An exemplary collar mechanism is described in U.S. Pat. No. 5,507,243, "CONNECTOR FOR UNDERWATER CABLES," Apr. 16, 1996. The locking bolt is adjustable between locked and unlocked positions as part of a latch mechanism. The locking bolt and other parts of the latch mechanism reside wholly or partly in cavities in the housing of the depth control device. Internal surfaces of the housing serve as bearing surfaces for the various elements of the latch mechanism. A typical latch mechanism is described in U.S. Pat. No. 5,214,612, "SWING PLATE LATCH MECHANISM," May 24, 1993.
As the streamer is towed through the water, it often happens that jetsam and other debris collide with the devices attached to the cable. Depending on the size of the debris and the tow speed of the vessel, the impact on the devices can be severe. Strong impacts can also result during payout and retrieval of the streamer from collisions with structures on the vessel. These impacts can exert strong forces on the devices that can deform or even fracture the housings. Damaged housings can result in seawater intrusion into the electronics in the devices, binding of the wings, or broken latch mechanisms. In these instances, the housing must clearly be replaced. Even if the force of impact causes only a slight deformation of the housing, the accuracy of the heading sensor contained within can be compromised if the deformation changes the relative alignment of the housing to the cable. A deformation causing even a 1.degree. or smaller change in alignment may be unacceptable in certain seismic applications. Typically, the solution is to replace the deformed device with an undeformed device. Damaged or deformed housings typically cannot be repaired and must be discarded. Even though the internal electronic components can be salvaged, they must typically be removed, reinstalled, and calibrated at the factory. Thus, it is clear that damaged or deformed housings can add significantly to the cost of outfitting and maintaining a streamer cable.
Clearly there is a need for avoiding permanent damage to expensive parts and for making field repairs possible in the inevitable event of collisions between cable-attached underwater devices and shipborne structures or underwater debris.